The present disclosure concerns a method of controlling fuel flow in a gas turbine engine. It also concerns a fuel flow control system and a fuel flow system for a gas turbine engine, which implement the method.
In a gas turbine engine it is conventional to control the fuel flow via a series of control laws. The amount of fuel flow may be determined by a function which is dependent on an engine shaft speed, a pressure ratio or a combination of these parameters. It may also be modulated by other factors such as altitude, for a gas turbine engine powering an aircraft. The fuel flow may be controlled to open loop, without feedback as to whether the instructed fuel flow has resulted in the desired engine speed change, or closed loop, with feedback.
Typically there are maximum and minimum fuel flow limiters which override the fuel flow control laws to ensure the engine is neither starved of fuel nor over-fuelled and hence flooded or caused to run away.
One problem with relying on the normal fuel flow control laws occurs when a gas turbine engine surges. In a surge the compressor discharge pressure drops rapidly regardless of the amount of fuel supplied to the engine. There is thus a danger of over-fuelling the engine during a surge. It is known to apply a surge control law so that fuel flow demand is pegged to the compressor discharge pressure, that is it is proportional to compressor discharge pressure, as it drops from detection of a surge condition. FIG. 2 shows this behaviour. The line NL_D shows the unaltered speed demand for an exemplary one of the engine shafts and line NL_A is the actual speed of the shaft. A surge occurs at approximately time 0.3. The line P30 shows the compressor discharge pressure which drops rapidly following the surge event. The line Wf_D shows the demanded fuel flow which has a small increase as the actual shaft speed NL_A drops as the control laws try to return the actual shaft speed NL_A to the demanded shaft speed NL_D. Then the demanded fuel flow Wf_D drops away tracking the reduction in compressor discharge pressure P30. The line is stepped due to the time constant of the applicable control law. The actual fuel flow Wf_A tracks the demanded fuel flow Wf_D with a lag.
When the compressor discharge pressure P30 begins to recover, at approximately time 0.6, the demanded fuel flow Wf_D also begins to increase. The surge is recovered where the compressor discharge pressure P30 and actual shaft speed NL_A plateau, since at this level the actual fuel flow Wf_A is sufficient to sustain the current actual shaft speed NL_A. However, because the fuel flow demand Wf_D is lower than the actual fuel flow Wf_A the fuel flow continues to reduce and thus the recovered engine continues to decelerate.
The increase in demanded fuel flow Wf_D is limited by a maximum rate limiter and so the increase in fuel flow, and consequently the arresting of the actual shaft speed NL_A decrease, is slow. Indeed, as illustrated the demanded and actual fuel flow Wf_D, Wf_A have only recovered to around 30% of their maximum values by time 1.4 when the speed demand NL_D is significantly reduced whereas before the surge, at the left hand side of FIG. 2, the demanded and actual fuel flows Wf_D, Wf_A were over 90% of their maximum values.